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METHYL ALCOHOL

41

25'5 grams of iodine in a solution of 50 grams of potassium iodide in 200
c.c. of water and making the volume up to i litre.

(d)  Approximately  N/io-sodium thiosulphate  solution,  prepared by
dissolving 25 grams of the pure salt to i litre.

(e)  Fresh starch paste.

TITRATION OF THE THIOSULPHATE SOLUTION. To 20 c.c. of an aqueous
solution containing 3-863 grams of potassum dichromate per litre are added
10 c.c. of 10% potassium iodide solution and 5 c.c. of hydrochloric acid
(D i-io). After mixing, 100-150 c.c. of water are added and the free iodine
titrated with the thiosulphate solution. Towards the end of the titration,
a little starch paste is added, addition of the thiosulphate being discon-
tinued when a drop of the latter changes the greenish blue colour to pale
green. Since 20 c.c. of the dichromate solution set free 0-2 gram of iodine
from the potassium iodide, the amount of iodine corresponding with i c.c.
of the thiosulphate solution may be readily calculated.

TITRATION OF THE IODINE SOLUTION. 10 c.c. of the iodine solution are
pipetted into a dish and the thiosulphate solution run in from a burette
until the liquid becomes pale yellow. Starch paste is then added and the
titration continued until the liquid becomes colourless. The amount of
iodine in 10 c.c. of the solution is found by multiplying the number of c.c.
of thiosulphate used by its iodine equivalent.

METHOD OF WORKING. 25 c.c. of the alcoholx are introduced into a litre
flask containing about 500 c.c. of water and the solution made up to volume.
After mixing, 10 c.c. of the liquid (corresponding with 0-25 c.c. of the methyl
alcohol) are placed in a 300-400 c.c. bottle fitted with a ground stopper
and containing 10 c.c. of the caustic soda solution (a) ; after mixing, 40
c.c. of solution (c) are added from a burette. The closed bottle is left for
an hour with occasional shaking, after which 10 c.c. of solution (b) are
added and the non-combined iodine titrated with the sodium thiosulphate
solution > (d). Subtraction of non-combined iodine from the amount
contained in 40 c.c. of the iodine solution gives the amount reacting.2
If the latter is a grams, the percentage of acetone is given by the
formula,

58^05 X a
761-52

X 100.

This gives the number of grams of acetone in 100 c.c. of the methyl
alcohol; multiplication by 0-7966—the density of acetone at 15°—then
yields the percentage of acetone by volume.

7. Bromine Absorption.—From this determination the quantity of
various impurities, principally allyl alcohol, is deduced. The reagents
required are :

(a] A solution of potassium brornate and bromide, prepared thus :
Powdered potassium brojnate and bromide (puriss.) are dried separately
in porcelain dishes for 2 hours at 100°. 2-447 grams of the bromate and

* With rectified methyl alcohol, 50-100 c.c. are taken according to its supposed
acetone content.

2 Each rnol. of acetone (58-05) requires 6 atoms of iodine (761-52). the pure